1. Field of the Invention
The invention is directed to a plasma treatment apparatus.
2. Description of Related Art
Plasma treatment apparatuses are used in surface treatment of workpieces for inserting foreign atoms into the workpiece surface, e.g. for the purpose of curing, reduction of wear or coating. In plasma treatment of workpieces, a glow discharge in a vacuum is generated between the workpiece and a counterelectrode while a treatment gas is present in the container at a low pressure (in the range of a few millibars only). From the plasma generated in the treatment gas during glow discharge, gas atoms diffuse into the surface layer of the workpiece.
In thermochemical surface treatment, e.g. in plasma cuburizing, the energy fed to the workpiece by the surface treatment processes has to be kept at a certain level by heat-transmission mechanisms. This mans that a specific treatment temperature, which is determined by the type of the respective procedure, must be maintained. To this effect, it may be necessary to withdraw heat through the container wall.
In a plasma treatment apparatus known from U.S. Pat. No. 4,645,981, the workpiece is inserted into a vacuum-tight container, and clocked glow discharges are generated between the container wall and the workpiece. According to the respective process to be carried out, the energy densities can vary from just a few microwatts to several watts per square centimeter. The container is enclosed by a heat-insulating sheath, and a cooling agent, having been introduced into the intermediate space, is sucked off in downward direction by a sucking means.
Plasma treatment systems have been used heretofore only for one process or, respectively, only for one specific workpiece surface. For this purpose, the container has been provided with a water jacket for cooling, with a heat insulation means provided inside. If such a container, to be used e.g. for a specific workpiece surface, id designed for a temperature of up to 1000.degree. C., the insulation of the container is much too high in case of an operating temperature of 500.degree. C. If the workpiece surface is small, the container can be used neither at 500.degree. C. nor at 1000.degree. C. because the energy yielded by the plasma is not sufficient for heating the container, i.e. an additional heating means has to be installed. The simple solution of using a container of high-quality material (e.g. Inconel) and --if needed--cooling it from the outside is not possible at temperatures above 800.degree. C. due to the lack of rigidity of the material. Another solution provides using a water-cooled outer container, mounting an insulating means of inferior quality and compensating the losses by an additional heating means. Although this solution is feasible, it is highly energy-consuming.
A plasma treatment apparatus known from British Patent No. 1 567 929 comprises a vacuum-tight outer container accommodating therein the inner container forming the negative electrode. An electric heating means is arranged in the intermediate space between the outer container and the inner container. The outer container can be provided with a water cooling means Also, a double-walled walled container of this type is always adapted only for a very restricted range of treatment temperatures.
British Patent No. 1 575 939 discloses a similar plasma treatment apparatus wherein, in a double-walled container, a heating means is arranged around the negative electrode and is surrounded by a radiation insulating means. The radiation insulation means consists a lamellae adjustable relative to each other.
It is an object of the invention to provide a plasma treatment apparatus which in case of high treatment temperatures, can be operated without a complicated cooling system and which, if required, can be used also for processes performed at low operating temperatures.